Dockerfile: Populate a pip cache

Given the number of jobs in CI we have which use python and pip install
packages, we should do this once in the Dockerfile, in order to populate
the cache. We let each job continue to create and use the virtual
environments they need to facilitate making updates to these
environments easier.

Signed-off-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Simon Glass <sjg@chromium.org>